Over-the-road trucks and other vehicles, such as school buses, often include a feature for limiting the maximum speed that the vehicle can travel. This feature, commonly referred to as a vehicle speed governor (VSG) or road speed limiter (RSL), is implemented in software that allows the user to program the maximum vehicle speed, e.g., for improving fuel economy, lowering maintenance costs, extending vehicle life and the like. Some systems include the ability to program separate maximum road speeds for different transmission gears.
In vehicles with automatic transmissions, a transmission controller controls operation of the transmission as a function of parameters such as transmission output shaft speed (TOS) and percentage engine torque (PET), which is the ratio of actual engine torque to maximum engine torque. The transmission controller controls the transmission using predetermined shift schedules or maps, which can be in the form of look-up tables that define shift points based on TOS and PET.
FIG. 1 illustrates representative shift schedules that can be implemented by a transmission controller for upshifting and downshifting the transmission between 4th and 5th gears. Similar shift schedules are provided for the other transmission shift points. The transmission controller uses the shift schedules to determine the appropriate transmission gear based on the current TOS and PET. If the intersect of these points falls below the 5-4 downshift curve, the transmission operates in 4th gear. When the intersect moves above the 5-4 upshift curve, the transmission controller sends a signal to the transmission causing it to upshift to 5th gear. Similarly, if the vehicle is operating in 5th gear and the intersect of TOS and PET drops below the 5-4 downshift curve, the transmission controller sends a signal to the transmission causing it to downshift from 5th gear to 4th gear.
Undesirable shuttle shifting, that is shifting back and forth between two gears, can occur when a vehicle is traveling at the RSL and the combination of engine torque and output shaft speed is on the gear shiftpoint. For example, assume RSL for 5th gear is set at 45 mph and the vehicle is traveling down a flat road at RSL speed. When the vehicle encounters an uphill grade, even a small grade, torque will increase if the driver maintains the vehicle speed at the RSL. As a result, the PET will increase, which may cause the vehicle to downshift, for example, from 5th to 4th gear. As the grade ends, torque decreases to maintain the given road speed, which may cause the transmission to upshift back to 5th gear. Such transmission shuttle shifting can be undesirable for drivability, for example, passenger comfort, and vehicle performance.